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Monday, February 25, 2013

Refractive Errors of the Eye: Basic Pathology of the Refractive Errors (Myopia, Hypermetropia, Astigmatism, Presbyopia)

Refractive errors: These are disorders of the eye, not diseases.

Emmetropia: A normal eye with no refractive errors is an emmetropic eye. Emmetropia means when parallel rays of light from infinity distance falls on the eye, it can focus them on the retina without any accommodation  Infinity distance in Ophthalmology is defined to be a distance of 6 m or more.
Emmetropia / Normal Vision
Emmetropia / Normal Vision

Ametropia: An eye with refractive errors when viewing objects at infinity distance is an ametropic eye. This eye has to accommodate the parallel rays of light to focus them on the retina.
Refractive Errors of the Eye
Refractive Errors of the Eye

Types of refractive errors / Ametropia:

  1. Myopia (Near-sightedness)
  2. Hypermetropia / Hyperopia (Far-sightedness)
  3. Astigmatism
  4. Presbyopia (Age related far-sightedness)

Basic Pathology of Refractive Errors:

1. Length of eye:

Normal Length of the eye is about 23 mm. The length is measured by A-scan ultrasound biometry.

Myopia: If the length of eye is too long then the image will fall in front of retina. This is called Myopia.

Hypermetropia: If the length of eye is short then the image will form behind the retina. This is called hypermetropia.

2. Curvature of cornea:

A. Myopia:
 If corneal curvature is more
More bending of light
Image will form in front of retina
 B. Hypermetropia:
If corneal curvature is flat
Less bending of light
Image will form behind retina

C. Astigmatism:
If corneal curvature is not perfectly spherical (one median more curved than the other)
Bending / refraction of light occurs in different orientation
2 focal images will be formed


Refractive errors can be corrected with spectacles, contact lenses or LASIK surgery.

That's all for today!
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